Geochemical and carbon isotopic studies of carbonaceous phyllites from Dharwar craton, India – Reconstruction of Precambrian depositional environment

Geochemical and organic carbon isotopic (δ13Corg) signatures of carbonaceous phyllites from Archean Chitradurga, Gadag, Sandur, Shimoga greenstone belts and Proterozoic Cuddapah basin of Dharwar Craton were studied to understand the Precambrian depositional environment by delineating the similarities and differences in the Archean and Proterozoic samples. Both Archean and Proterozoic carbonaceous phyllites are fine grained siliceous in nature with depleted CaO, MgO and alkalies. The REE concentrations in most of the Archean samples are characterized by conspicuous positive Europium anomalies while the Proterozoic counterparts display feebly positive ratios indicating hydrothermal source. The Archean samples possess elevated Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Weathering (PIA) values reflecting extreme weathering conditions while the Proterozoic carbonaceous phyllites are confined to moderate weathering conditions. Cr/Th, La/Sc, Th/Sc and Co/Th indicate mafic provenance and the Cr-Zr relationship suggests deep sea depositional setting for both Archean and Proterozoic carbonaceous phyllites. The Archean carbonaceous phyllites indicate humid to semi-humid paleoclimate conditions while the Proterozoic samples are restricted to humid palaeoclimatic conditions which is evident from Sr/ Cu ratios. V/(V + Ni) ratios indicate fluctuating dysoxic to euxinic redox conditions for the Archean samples while the Proterozoic samples display euxinic conditions. The predominantly negative δ13Corg values of Chitradurga (− 38.5‰ to − 14.2‰ VPDB), Gadag (− 24.3‰ to − 8.06‰ VPDB), Sandur (− 24.8‰ to − 29.2‰ VPDB), Shimoga (− 24.6‰ to − 11.8‰ VPDB) and Cuddapah (− 30.19 to − 13.98‰ VPDB) suggest the role of primitive life forms through biological processes during the deposition of organic matter. 

Figure: Contribution of seawater



C.S. Sindhuja, C. Manikyamba, Subhojit Saha, Siva Narayanan, B. Sridhar (2022) Precambrian Research 372, 106575, external link